The present invention relates to a thermal recording system.
Along with the development and popularity of data processing apparatus such as personal computers and word processors, recording apparatus have increased in importance as output terminal devices. Various types of recording or printing apparatus such as wire-dot, ink-jet and thermosensitive recording apparatus have been developed. Among these, a thermal recording apparatus is receiving most attention these days. The thermal recording apparatus has advantages in that a normal paper sheet can be used for recording, a noise level at the time of recording is low, a recording mechanism is simple, easy maintenance can be performed, and alteration of recorded data is impossible. Furthermore, along with the development of color output terminal devices such as a color CRT display, there arises a demand for a color recording apparatus. A color thermal recording apparatus can be easily arranged to perform good color reproduction. The color thermal recording apparatus is the most promising apparatus among various types of color recording apparatus.
In the conventional thermal recording apparatus, a number of thermal heating resistive elements are aligned in line, and the thermal heating resistive elements are selectively supplied with current in accordance with a recording signal. This energizing cycle is then repeated to heat the resistive elements, so that an ink carried on an ink ribbon is melted by the heated resistive element. The ink is then transferred to the paper sheet so as to record an image on the sheet. Although the thermal recording apparatus has the above advantages, it has a drawback in that the recorded image becomes poor due to a heat retention or storage effect of the resistive elements as the recording speed increases. In order to increase the recording speed, an interval between energizing cycles is shortened. When a resistive element which was energized in the immediately preceding energizing cycle is reenergized after a short time interval, heat cannot be sufficiently lost. Therefore, when the same resistive element is successively energized at a short interval after the immediately preceding energizing cycle, the temperature of this resistive element continues to increase. In this manner, when the energizing cycles are repeated at short intervals, the present temperatures of the individual resistive elements differ due to their thermal history. When resistive elements having different temperatures are simultaneously energized, areas at which inks are melted differ, thereby resulting in an image having a nonuniform density. In particular, when characters are recorded, the ink is often transferred to a narrow space which does not correspond to the image data, thus degrading legibility.
In order to solve the above problem, a system is proposed in Japanese Patent Publication No. 55-48631, wherein an energizing time of each thermal heating resistive element when mark data as recording data are continuously supplied is set to be shorter than that when the mark data follows space data. The energizing time of the subsequent energizing cycle of a given thermal heating resistive element is switched in a two-step manner in accordance with whether or not the given resistive element was energized in the immediately preceding energizing cycle. According to this system, the above drawback can be eliminated to some extent. However, in fact, this system cannot eliminate the nonuniform density of the recorded image since the thermal histories of the resistive elements still differ from each other, especially in high speed recording, due to insufficient controllability.